A Complete Guide to Street Supercharging

By Pat Ganahl

The photos in this edition are black and white.

Once the limits of a naturally aspirated engine are achieved in terms of horsepower and reliability, there's only one more way to maximize horsepower potential: forced induction. There are two options for realistic forced induction, a turbocharger or a supercharger. While there is considerable debate over which is better, both offer exponential gains over any standard modification on a normally aspirated engine.

Street Supercharging, from industry veteran Pat Ganahl, has been the guidebook for supercharging fans for years. As time and technology march on, updates are required to keep things current (as of 2009) and that's what this revised edition of Street Supercharging does. It covers blower basics, blower background and history, a tutorial on how blowers work, information on used superchargers and their practicality, chapters on the different styles of superchargers, like the traditional roots style blowers vs. the emerging centrifugal styles, blower installation, how to build your engine to handle the demands of a blower application, and information on tweaking factory blower systems.

• Language: English
• Publisher: S-A Design
• Release date: Aug 14, 2020
• ISBN: 9781613256756

Book preview

CHAPTER 1

BLOWER BASICS

No single performance modification to an internal combustion engine is more practical or effective than the addition of a supercharger.

That was the opening statement of this book when it was first written in 1984, and again when it was revised in 1999. Now, as we are well into a new millennium and the automobile is celebrating its second century of production, that statement is as true as ever. It is even more relevant to performance vehicles of all types, whether they are built on an assembly line or modified in your local speed shop or home garage.

That might sound like technological bragging or advertising hype, but you’d get little objection from engineers, engine builders, or race car drivers. The supercharger’s effectiveness has been known almost since the engine was invented.

The audio-visual effects of a big, shiny, whining supercharger are sure to draw a crowd and admiring glances at the local cruise night, especially when it’s out in the open, not under a hood. The 302 small-block in this early Mustang doesn’t need a 6-71 with two 4-barrels, but note the large pulley on the blower to underdrive it. Belt-driven blowers are easily adjustable to make them very drivable, and the best part is that they work even better than they look.

So why haven’t auto manufacturers put blowers on all cars? It has always been a matter of economics. Buyers of mass-produced transportation vehicles have always been more motivated by bottom-line cost than performance and, until recently, the cost of gasoline in America has been so low that little emphasis had ever been put on significantly increasing engine efficiency. If you want more power, make the engine bigger! Well, we’re finally learning that adding a supercharger to a smaller engine can do the same thing. Manufacturers and typical buyers are also learning that smaller, more-nimble cars are not only more efficient, but they also perform much better than the big, lumbering vehicles of a couple decades ago—or more recent SUVs and crew-cab pickups. Yes, today more and more new-car manufacturers are supplying vehicles with superchargers of several types.

Better yet, those of us who are of the hot rod or do-it-yourself automotive-performance-enhancement persuasion can now choose from a widening variety of supercharger types and sizes. New superchargers have significantly improved power/efficiency accessories such as inter-coolers and electronic fuel injection (EFI), to either immediately pump up the muscles of an undernourished small or medium-size motor, or put a big, already muscular motor on an amazing-to-ridiculous dose of steroids.

The Air Engine

Before we can even begin to discuss what a supercharger is, what it does, and why your street-driven engine should have one, we need to dispel a major myth about the internal combustion engine itself.

You probably think the engine in your car runs on gasoline. Most people do. But it doesn’t. It runs on air—expanding air. That’s what pushes the pistons and turns the crank. All the gasoline does is make a very hot fire to heat the air. Heating the air makes it expand.

A 16-plug, dual distributor, 426-inch Chrysler Hemi is a plenty impressive piece of machinery, but a big, polished, 6-71 GMC supercharger adds a whole new dimension to such a motor, figuratively as well as literally in terms of its air-capacity volumetric size—and power.

While a supercharger can make a big engine bigger, the real beauty is that it can make a smaller engine act like a big one. Obviously, the blower size should be matched to engine capacity. This well-manicured example is a 110-ci early Magna Charger on a 231-ci Buick V-6.

You probably think of the fire in the cylinder, when the spark plug ignites the compressed air/gasoline mixture, as an explosion, and that this powerful explosion blows the piston down the cylinder on the power stroke. Well, this explosion is nothing more than very rapidly expanding air (which has been rapidly and extremely heated). That’s what an explosion is, by definition: very rapidly and forcefully expanding air, which blows things apart. The word is more appropriate than you might have thought.

There are two primary points to consider here (we’ll get more technical later…which you can skip if it doesn’t interest you). The first is that the size of an engine is measured in cubic inches (ci). Cubic inches of what? Air. Let’s say you have a 300-ci engine and it has 8 cylinders. That means each cylinder has a volume of 37.5 ci when the piston is at the bottom of its travel. If this engine had 100-percent volumetric efficiency (VE), the cylinder would draw in a total of 37.5 ci of air on each intake stroke. Through one full cycle of the firing order, this engine would draw in, breathe, or consume 300 ci of air in its eight cylinders.

We don’t need to get more technical than that right now. What you know is that the bigger the engine—the more total cubic-inch volume (or displacement) of its cylinders, the more powerful it is. We all know that. In fact, we know that, in general, a 400-ci engine is about twice as powerful as a 200-ci one, other things being the same. So the power of an engine is based on the amount of air it holds, processes, or consumes, not the amount of fuel it uses. In fact, if you know anything about tuning gasoline engines, you know that if you try to add too much fuel to the mixture—that is, make it run rich—it will start losing power, significantly. So, adding more fuel to an engine doesn’t make it more powerful; adding more air to it does. You can probably see where we’re going with this.

The second point, which can get complicated, is that air, like any gas, is elastic. It expands and contracts, and it can be compressed. This is critical to supercharging. In fact, it’s what the whole process of supercharging is based on.

This Nostalgia Top Fuel dragster demonstrates two things: first, drag racers have known since the 1950s that blowers work, big time. Second, you can readily see from the gaping injectors that the blower’s job is to gulp a whole lot of fresh air and pump it into the motor to make more power. Even limited to small 6-71 GMC blowers, these cars run 250+ mph with obsolete Chrysler engines.

To begin at the most basic, all the stuff in the world—technically known as matter—is made of atoms and molecules. And most of this matter can exist in one of three forms, depending on its temperature: solid, liquid, or gas. Solids can be melted into liquids, and liquids can be boiled (or evaporated) into gases, and vice versa. At normal atmospheric (or room) temperatures, certain substances exist as solids, some as liquids, and some as gases. Even though we usually can’t see it or feel it, air is a gas. It is matter made up of atoms and molecules. The air that we breathe, as you know, contains about 21 percent oxygen. Another 78 percent or so is nitrogen. The remaining 1 percent contains obviously small amounts of argon, carbon dioxide, methane, etc. And that’s if the air is clean and dry.

But the point is that air is matter; it’s made of stuff (atoms and molecules) and it has weight (or mass as the physicists call it). If you don’t believe it, consider that air can mess up your hair, it can knock you down, or it can even rip the roof off your house if it’s blowing hard enough. Air can be quite forceful. It has mass, even though you can’t see it. The dynamics of air moving inside your engine are a bit different than those of wind in a hurricane, but it’s the same stuff doing the work. Air packs a punch. It’s what makes the horsepower in your engine and moves your car. That’s what we’re talking about.

We said that air has weight. Since this weight presses on everything at the earth’s surface equally, from all sides (like the weight of water at the bottom of a pool or the ocean), we measure it as pressure—so much weight per square inch. You probably know that the weight of air varies with its temperature. Warmer air is lighter (less dense) than cold air. This has a lot to do with supercharging, and we’ll discuss it much more in Chapter 3. But for now, understand that this is what we call barometric pressure and differences in air pressure (due to temperature) between one place and another is what causes wind—sometimes very strong wind. You probably also know that the higher up you go in altitude, the thinner (that is, lighter) the air gets. That makes sense, since the higher you go from the earth’s surface, the less air there is above you pressing down.

OK, the point of this little lesson is that the normal, or average, pressure of air at the earth’s surface is about 14.7 pounds per square inch (psi). Since this amount is considered normal, most air pressure gauges start at this amount as zero, and read increases in air pressure above that. Therefore, a reading of 14 to 15 psi on a typical air pressure gauge is really about 28 to 29 pounds of actual air pressure (which scientists call absolute pressure; the reading on the gauge is called gauge pressure). That means if an air pressure gauge reads about 15 psi, whatever it’s hooked up to—say the tank on your air compressor—contains about twice as much air as it did when the gauge read 0 psi. In other words, if your air compressor has a 5-gallon tank, and you run it until the tank gauge reads about 15 psi, then that tank has the equivalent of 10 gallons of air in it. You can’t put 10 gallons of water in a 5-gallon tank, just like you can’t put 50 pounds of solid, um, stuff in a 5-pound bag, because liquids and solids won’t compress. But air, like any gas, will.

And if anything worked better than Roots-style blowers, today’s Top-class dragsters and Funny Cars would use it. But nothing does. They’ve tried. Even limited to 14-71 length and 60-degree helix, GMC-type blowers now push these still-obsolete Chrysler-based engines to well over 300 mph with something like 3,000 hp.

There’s no question GMC-style blowers work, but there are lots of types of superchargers, and several of today’s are more refined and more efficient—and don’t have to stick out of the hood. Some say the downfall of centrifugal blowers is that they don’t make low-end power. But looked at it another way, their power output increases geometrically with blower speed. A setup like this Vortech blowing into an aftermarket EFI throttle body through an intercooler can produce upwards of 1,200 hp from this Duttweiler small-block, in a hurry.

The point we’re making about engines and superchargers should be obvious by now. The internal combustion engine runs on air. The more air it holds (the bigger the displacement), the more power it makes. And a supercharger can pump more air into the engine than the natural atmosphere can—lots more.

Engines like these look impressive and can run strong in a high-RPM power range. But they run very poorly at street-driving speeds, are difficult to tune and maintain, and can actually cost considerably more to build—especially if trick heads are included—than simply bolting on a supercharger. Blowers are more practical and efficient, especially for the total price.

Yes, but we hot rodders, like Detroit engineers, sometimes think too traditionally. We know that we can add more or bigger carburetors, increase the size of the ports, or hold the valves open higher and longer with a bigger camshaft to make more power in a given engine. Why? To get more fuel in? No, to get more air into the cylinders. Once we get more air in, then we have to add more fuel to keep the air/fuel ratio (fuel mixture) correct. We also know that we can bore and stroke the engine to make it hold more air, and thus make more power.

Before going any further, let’s dispel another engine myth. You’ve undoubtedly heard the old saw that an internal combustion engine is basically an air pump. I’m not sure where that analogy came from or what it’s really supposed to mean, but it’s 180-degrees wrong. An internal combustion engine doesn’t pump air, it sucks air. That’s its basic problem, which a supercharger—which is an air pump—can quickly and easily solve.

The problem is that a naturally aspirated engine has only the outside air pressure to push air into the cylinder when the piston moves down on the intake stroke. Looking at it the other way around, the piston moving down in the bore creates a vacuum, which sucks the outside air into the cylinder. (That’s just two different ways of saying the same thing.) But it has to suck that air past an opening valve, through a long port, through a usually convoluted intake manifold, past the venturis of a carburetor (or through a throttle body), and through a restrictive air cleaner of some sort. And the faster the engine turns, the less time it has to do all this.

If a 300-ci engine could fill its cylinders with 300 ci of air on each full intake cycle, that would be called 100-percent volumetric efficiency (VE). But you can see why most naturally aspirated engines don’t come close to 100-percent VE.

But we traditional (dare I say, stubborn?) hot rodders have been porting and relieving, adding more or bigger carbs and manifolds, grinding bigger cams, even boring and stroking for bigger displacement—just to try to fill the engine with as much air as it will hold. That is, to try to get closer to 100-percent volumetric efficiency. Given natural air pressure, that’s the best you can do. That’s the limit.

So why not just bolt some sort of air pump on the engine and force more air into it? Bingo! It doesn’t really take a genius to figure this out. That pump is called a supercharger, and it’s the simplest, most-effective single piece of equipment you can add to your engine to dramatically increase its efficiency and power.

Yes, a supercharger might be the single most expensive piece of equipment you can add to your engine, but compare it to all the work and expense of traditional modifications it can not only replace, but actually supersede. Adding a mild-boost (5 to 7 psi) blower to any stock engine can easily achieve or exceed 100-percent VE without any other modifications. Such boost levels won’t tax the engine mechanically, and will keep it tame and very tractable on city streets or the highway. As already mentioned, increasing blower boost (by a simple pulley swap, in most cases) to 14 to 15 psi is effectively the same as doubling the size—and power—of your engine. In the case of most modern powerplants the only limiting mechanical factor to doing so is the available-gasoline’s octane rating.

Or, if you’re a typical hot rodder who always wants more power, supercharging is really the only way to do it in a given engine. Yes, you’ll have to make traditional modifications to help the engine breathe the extra air you’re forcing into it, and to make parts like pistons and rods withstand the extra force it will create. But boost levels up to 30 psi aren’t out of the question, especially for short bursts of speed, which is how most blowers are used.

Although it took hot rodders a while to appreciate what blowers could do, they started to experiment with them shortly after World War II, as this GMC 2-71, cleverly and cleanly adapted to a Willys Jeep, attests.

Look at it this way. The ultimate, most unrestricted form of automotive competition—and by far the most powerful—is the Top Fuel class in drag racing. And every one of those cars uses a big, belt-driven supercharger. Need we say more?

Street Supercharging

The main reason superchargers of some sort aren’t used in all forms of racing is that rules regulate against it. But there are no such rules for street-driven vehicles.

In the following pages we will discuss some of the history and theory of supercharger development, but we will be primarily interested in showing you the several varieties of superchargers and complete kits currently available for street performance engines, how they should be installed and maintained, and how the package can be tuned for maximum performance and efficiency. We will also study the types of changes that should be made to a typical street motor (and those that shouldn’t) before a supercharger is installed, as well as a rundown on the best ancillary components for both the engine and the blower (drive systems, linkage, ignition systems, camshafts, exhaust systems, intercoolers, and so on).

While we don’t really recommend buying a used blower at a swap meet (or unseen over the Internet), given the strong interest in vintage engines and equipment these days, we will give an overview of the wide variety of superchargers that have been available on production automobiles as well as through the aftermarket over the past several decades. Plus, surprisingly, we will even look at some brand-new and improved reproductions of these early blowers that are now available.

And while the current trend, given today’s proven and efficient electronically fuel-injected engines, is toward centrifugal, screw-type, or smaller and more-efficient Roots blowers integrated into an EFI system—especially with a highly effective intercooler—we will see that there are still plenty of ways to supercharge your engine. These include a big, polished, 6-71 or 8-71 Jimmy sticking through the hood, Gasser-style, with either a pair of 4-barrels on top or a Hilborn- or Enderle-style injector converted to programmable electronic operation. But no matter which approach you take, as long as you follow the several hints and guidelines offered here to get the system matched and tuned properly, you will certainly agree that no other single performance upgrade can compare with supercharging.

But even hot rodders were a bit incredulous when Tom McMullen’s blown, flamed, street-driven 1932 roadster hit the cover of Hot Rod magazine in 1963. It won class at the NHRA Winternationals, set lakes records at El Mirage, raced on the street and strips every weekend, yet still drove to work. Superchargers could do that then, and they can do it now.

What is a Supercharger?

If we are going to spend the rest of this book talking about superchargers, we had better start by determining exactly what a supercharger, or blower, is. In the broadest sense, a supercharger is anything that will force more air into the cylinders of an engine than would be drawn into the cylinders naturally by the suction of the pistons during the intake stroke. At sea level, atmospheric pressure is approximately 14.7 psi. Air exerts this much naturally on everything near the surface of the earth (because of the air’s weight). When an engine is naturally aspirated (not supercharged), it must rely on this pressure to push the air into the carburetor or throttle body, through the manifold, into the intake port, past the opening valve, and then into the cylinder as the piston opens the cylinder to maximum volume when it moves downward on the intake stroke.

We’ve touched on volumetric efficiency, but let’s study it further, because it’s the natural law of physics the supercharger can break, and thus do more good for your engine than any other mechanical modification. Because of the restrictions and bends along the path, friction, turbulence, incomplete evacuation of the exhaust from the cylinder, and several other factors, the naturally aspirated cylinder is never able to completely draw in a full charge of air/fuel mix.

Let’s say the total volume of the cylinder is 40 ci. If you turned the engine by hand to bottom dead center (BDC) on the intake stroke, and left it there, the cylinder would fill with 40 ci of air through the open intake valve. Atmospheric pressure pushes the air in (which is exactly the same as saying the downward-moving piston sucks the air in). Whenever you create a space that has nothing in it—that is, a vacuum—atmospheric pressure will immediately push air into that space through any available opening. The force pushing the air in will be approximately 14.7 psi at sea level; less at higher altitudes. But when the engine is running at speed, atmospheric pressure is not great enough to push 40 ci of fresh air/fuel mixture into the cylinder before the intake valve closes and the compression stroke begins.

What we are describing is the volumetric efficiency of the engine, which is a comparison of the total amount of air a naturally aspirated engine could draw in (total cylinder volume), to the amount (volume) that it actually does draw in under given operating conditions. Other factors being equal, an engine’s power output is directly proportional to its volumetric efficiency, and the vast majority of our traditional hop-up tricks are directed at increasing this figure: carburetion (or injection), intake manifolding, porting, polishing, valve size, camshaft design, exhaust scavenging, and so on.

But no naturally aspirated engine can attain 100-percent volumetric efficiency (except in rare cases when a perfectly tuned ram-induction intake manifold can accelerate the intake charge sufficiently to ram it into the cylinder at a certain, narrow span in the RPM range). By increasing intake tract and port sizes, streamlining passages, and evacuating exhaust, we can reduce losses and increase the efficiency to a certain point. But as long as we are relying on atmospheric pressure to push the air/fuel charge into the cylinder—if that remains constant—we can very rarely (if ever) reach 100-percent efficiency.

After you have done everything you can to help the engine draw as much air into the cylinders as possible by normal suction, there remains only one way to get more air/fuel charge in—and it’s a simple deduction: increase the pressure pushing the air into the motor; force the air in; and pump the air in. That is what a supercharger is—an engine air pump.

So in the broadest sense a supercharger is any device or means for increasing the cylinder-filling (volumetric efficiency) in the engine beyond that possible by the normal